DNA methylation of the mouse middle repetitive A particle genes and the mouse globin genes is being examined as a function of age. The method involves parallel digestions of mouse DNA with the restriction endonucleases Msp I and Hpa II, Southern blotting, and hybridization with nick translated probes. The present evidence indicates that some A particle sequences decrease in methylation during aging, while the entire genome in various tissues remains relatively constant in methylation between 86 and 31 months of age. In addition, a new RIA for 5meC has been developed, based upon inhibition of binding between radioactive 5 methyl cytidine and anti-5 methyl cytidine. This RIA will be used to assess age-related changes in overall methylation of germ cell DNA.